How to change the default disk space quota

Hello everyone!
My account is suppose to be capable of hosting "unlimited domains", but ever since i started adding domains in jan05 it has been counting down from 5.

I have been allocated 500MB of space and each new account defaults to a quota of 100MB each. I tried to add a 6th today but this message appeared: "Sorry you are at your limit for creating accounts (You have 5 accounts, your limit is 5)".

Now is this a product of 5 x 100MB = 500MB therefore I don't have room to add more accounts? If so, how can I reduce the quota allocation for each account? (I only use a 1MG or 2 for each account anyway)